Fix input event streaming behavior semantics

packages/coding-agent/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Fixed extension input events to report `streamingBehavior` only for prompts actually queued during streaming ([#5107](https://github.com/earendil-works/pi-mono/pull/5107)).
 - Fixed fenced `diff` code blocks and other highlight.js scopes to keep theme-aware syntax colors after the `cli-highlight` replacement ([#5092](https://github.com/earendil-works/pi/issues/5092)).
 
 ## [0.76.0] - 2026-05-27

packages/coding-agent/examples/extensions/README.md

@@ -75,6 +75,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `reload-runtime.ts` | Adds `/reload-runtime` and `reload_runtime` tool showing safe reload flow |
 | `interactive-shell.ts` | Run interactive commands (vim, htop) with full terminal via `user_bash` hook |
 | `inline-bash.ts` | Expands `!{command}` patterns in prompts via `input` event transformation |
+| `input-transform-streaming.ts` | Skips expensive input preprocessing for mid-stream steering via `streamingBehavior` |
 
 ### Git Integration
 

packages/coding-agent/src/core/agent-session.ts

@@ -984,7 +984,7 @@ export class AgentSession {
 					currentText,
 					currentImages,
 					options?.source ?? "interactive",
-					options?.streamingBehavior,
+					this.isStreaming ? options?.streamingBehavior : undefined,
 				);
 				if (inputResult.action === "handled") {
 					preflightResult?.(true);

packages/coding-agent/test/suite/agent-session-prompt.test.ts

@@ -5,6 +5,7 @@ import type { AgentTool } from "@earendil-works/pi-agent-core";
 import { fauxAssistantMessage, fauxToolCall, type Model } from "@earendil-works/pi-ai";
 import { Type } from "typebox";
 import { afterEach, describe, expect, it } from "vitest";
+import type { InputEvent } from "../../src/core/extensions/index.ts";
 import type { PromptTemplate } from "../../src/core/prompt-templates.ts";
 import { createSyntheticSourceInfo } from "../../src/core/source-info.ts";
 import { createTestResourceLoader } from "../utilities.ts";
@@ -259,6 +260,80 @@ describe("AgentSession prompt characterization", () => {
 		expect(getMessageText(harness.session.messages[0]!)).toBe("from extension");
 	});
 
+	it("does not report streamingBehavior to input handlers while idle", async () => {
+		const inputEvents: InputEvent[] = [];
+		const harness = await createHarness({
+			extensionFactories: [
+				(pi) => {
+					pi.on("input", (event) => {
+						inputEvents.push(event);
+					});
+				},
+			],
+		});
+		harnesses.push(harness);
+		harness.setResponses([fauxAssistantMessage("ok")]);
+
+		await harness.session.prompt("idle", { streamingBehavior: "followUp" });
+
+		expect(inputEvents).toHaveLength(1);
+		expect(inputEvents[0]?.streamingBehavior).toBeUndefined();
+	});
+
+	it("reports streamingBehavior to input handlers while streaming", async () => {
+		let releaseToolExecution: (() => void) | undefined;
+		const toolRelease = new Promise<void>((resolve) => {
+			releaseToolExecution = resolve;
+		});
+		const inputEvents: InputEvent[] = [];
+		const waitTool: AgentTool = {
+			name: "wait",
+			label: "Wait",
+			description: "Wait for release",
+			parameters: Type.Object({}),
+			execute: async () => {
+				await toolRelease;
+				return {
+					content: [{ type: "text", text: "released" }],
+					details: {},
+				};
+			},
+		};
+		const harness = await createHarness({
+			tools: [waitTool],
+			extensionFactories: [
+				(pi) => {
+					pi.on("input", (event) => {
+						inputEvents.push(event);
+					});
+				},
+			],
+		});
+		harnesses.push(harness);
+		harness.setResponses([
+			fauxAssistantMessage(fauxToolCall("wait", {}), { stopReason: "toolUse" }),
+			fauxAssistantMessage("done"),
+		]);
+
+		const sawToolStart = new Promise<void>((resolve) => {
+			const unsubscribe = harness.session.subscribe((event) => {
+				if (event.type === "tool_execution_start") {
+					unsubscribe();
+					resolve();
+				}
+			});
+		});
+
+		const promptPromise = harness.session.prompt("start");
+		await sawToolStart;
+		await harness.session.prompt("queued", { streamingBehavior: "followUp" });
+
+		expect(inputEvents.map((event) => event.streamingBehavior)).toEqual([undefined, "followUp"]);
+
+		releaseToolExecution?.();
+		await promptPromise;
+	});
+
 	it("throws when prompted during streaming without a streamingBehavior", async () => {
 		let releaseToolExecution: (() => void) | undefined;
 		const toolRelease = new Promise<void>((resolve) => {